Paget's disease (PD) is the second most common bone disease, affecting 1-2 million patients in the U.S. PD is one of the most exaggerated examples of normal bone remodeling and is a paradigm for studies of bone remodeling. Both genetic and environmental factors have been implicated in the pathogenesis of PD, and our current RO1 grant is examining the interaction of these two components in the development of PD in vivo. The primary cell involved in the pathogenesis of PD is the osteoclast (OCL). OCLs from PD patients have a distinct phenotype. They express measles virus nucleocapsid protein (MVNP), are hyper-responsive to 1,25-(OH)2D3 and RANKL, produce high levels of IL-6 and TAFII-17, a coactivator of VDR, and have an increased bone resorbing capacity per OCL. Importantly, targeted expression of MVNP to the OCL lineage in transgenic mice using the TRAP promoter (TRAP-MVNP mice) results in development of pagetic bone lesions and OCLs in these mice. However, the role that vitamin D receptor (VDR) hyperactivity plays in the development of PD is still unclear. We previously reported that TAFII-17 is induced by MVNP, and increased TAFII-17 levels are present in PD patients. Increased TAFII-17 levels result in hyper- responsivity of OCL precursors to 1,25-(OH)2D3. However, increased expression of TAFII-17 by itself is not sufficient to induce pagetic OCL in vitro but requires addition of IL-6 to the cultures. Importantly, MVNP cannot induce pagetic OCL in the absence of VDR. We found that binding of 1,25-(OH)2D3 to VDR in OCL precursors expressing MVNP increases activation of distinct signaling pathways (e.g. p38 and PLC32), which either increase IL-6 production, OCL formation or nuclear number per OCL. Objective/Hypothesis: It is our hypothesis that environmental factors implicated in the pathogeneses of PD, such as measles virus (MV), increase IL-6 levels in OCL precursors, which in turn increase TAFII17 expression. The increased expression of TAFII-17 enhances VDR activity at physiologic concentration of 1,25-(OH)2D3, that in turn activates both the p38 MAPK and PLC32/CREB pathways to increase IL-6 production, OCL numbers, nuclei per OCL and OCL bone resorption capacity. Further, elevated local levels of IL-6 induce normal OCL precursors to become hyper-responsive to vitamin D and form hyper- multinucleated OCL with increased bone resorbing capacity in the presence of vitamin D in vivo. To test this hypothesis we will pursue the following specific aims. Specific Aim 1: Determine if increased VDR activity in combination with high local levels of IL-6 in OCL precursors results in formation of hyper-multinucleated OCLs and increased bone resorption in vivo. Specific Aim 2: Determine if administration of vitamin D3 to TRAP-IL-6 mice results in formation of hyper- multinucleated OCLs and increased bone resorption. Study Design: Aim 1: We will characterize the effects of increased expression of TAFII-17 in the absence or presence of elevated IL-6 in OCL on pagetic OCL formation in vivo by analyzing TRAP-TAFII-17, TRAP-IL-6, and TRAP- TAFII-17/IL-6 (generated by breeding the first two lines) mice (followed over 18 months) for development of bone lesions, abnormal OCL activity, excessive new bone formation and other features of PD that we have reported in our TRAP-MVNP mice. We will compare these results to those obtained from TRAP-MVNP and WT mice. Aim 2: We will administer 1,25-(OH)2D3 (0.1 <g/kg IP per day) or vehicle daily to TRAP-IL-6 mice for 4 weeks, and determine if these mice develop pagetic OCL and bone lesions characteristic of PD in vivo by histomorphometry analysis as well as analysis of whole blood ionized calcium, phosphorus, alkaline phosphatase, and Pyd levels. Impact: This proposal will test for the role of increased VDR activity in the development of PD-OCL and bone lesions in vivo and if co-expression of increased levels of TAFII-17 and IL-6 in normal OCL precursors is sufficient to induce hyper-multinucleated OCLs characteristic of PD when mice are treated with vitamin D3.